Re: A lean way for getting the size of the instruction at a given address

Zied Guermazi
2021-04-05T16:17:31Z

hi Luis

A new member function in "class gdb_disassembler" to calculate the
instruction length only will be a good solution. In fact a big overhead
is added by the printing of instruction disassembly, which is not needed
at all. On aarch64, the decoder is optimized to issue many instruction
in one trace element, and here calculating the size consumes more than
80% of the time. On arm, the decoder issues one instruction after
another and here getting the size consumes 50% of the time. Considering
the amount of traces this can sum up to a dozen of minutes in some cases
(64MB of traces)

Calculating the instruction size per se, on arm is a "rapid" operation
and consists of checking few bits in the opcode. So the time can be
drastically decreased by having a function to calculate the size only.

gdb_print_insn can be then changed as following (pseudo code):

int
gdb_print_insn (struct gdbarch *gdbarch, CORE_ADDR memaddr,
struct ui_file *stream, int *branch_delay_insns)
{

gdb_disassembler di (gdbarch, stream);

if ( di.get_insn_size != 0)

return di.get_insn_size(memaddr);

else

return di.print_insn (memaddr, branch_delay_insns);
}

Is there a function in aarch64-tdep or arm-tdep doing job of disassembly
( the lower layer handling the opcode)? are we relaying on the bfd
library for it? can someone give me a hint of where to find those functions?

Kind Regards

Zied Guermazi

On 05.04.21 15:01, Luis Machado wrote:
> Hi Zied,
>
> On 4/4/21 4:59 AM, Zied Guermazi wrote:
>> hi
>>
>> I need to get the size of the instruction at a given address. I am
>> currently using gdb_insn_length (struct gdbarch *gdbarch, CORE_ADDR
>> addr) which calls gdb_print_insn (struct gdbarch *gdbarch, CORE_ADDR
>> memaddr, struct ui_file *stream, int *branch_delay_insns). and this
>> is consuming a huge time, considering that this is used in branch
>> tracing and this gets repeated up to few millions times.
>>
>>
>> Is there a lean way for getting the size of the instruction at a
>> given address, I am using it for aarch64 and arm targets.
>
> At the moment I don't think there is an optimal solution for this. The
> instruction length is calculated as part of the disassemble process,
> and is tied to the function that prints instructions.
>
> One way to speed things up is to have a new member function in "class
> gdb_disassembler" to calculate the instruction length only.
>
> Another way is to have a new gdbarch hook that calculates the size of
> an instruction based on the current PC, mapping symbols etc.
>
>>
>> Kind Regards
>>
>> Zied Guermazi
>>
>>
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